Saturday, July 26, 2014

Signature of a Giant Planet’s Rocky Core

Stars in binary systems generally have identical chemical
compositions since they formed from the same natal cloud of material.
Nevertheless, small differences in chemical composition can exist between a
pair of stars in a binary system and one explanation is the process of planet
formation. When planets form around a star, it can cause the star to be
slightly depleted in heavy elements (i.e. elements heavier than hydrogen and
helium) compared to its companion star.

Observations of 16 Cygni, a binary system comprised of two
stars 16 Cygni A and 16 Cygni B (hereafter components A and B), reveal that
component B has a giant planet with at least 1.5 times Jupiter’s mass. The
giant planet, identified as 16 Cygni Bb, orbits its host star in a highly-eccentric
800-day orbit. At its minimum and maximum distances from its host star, the
giant planet receives, respectively, 4.4 and 0.16 times the amount of
insolation Earth gets from the Sun. Being a giant planet, 16 Cygni Bb is
composed primarily of hydrogen and helium, much like Jupiter.

Figure 1: Artist’s impression of a giant planet with a
system of moons around it.

Figure 2: Differences in heavy element abundance between
components A and B versus condensation temperature. The dashed line is the
average of the volatiles and the solid line is the average of the refractories.
The dot dashed line is the mean trend for 11 Sun-like stars compared to the
Sun. Maia et al. (2014).

A study by Maia et al. (2014) show a small difference in the
abundance of heavy elements (i.e. metallicity) between components A and B of 16
Cygni. Component A has an overall metallicity (i.e. abundance of heavy
elements) that is 0.047 ± 0.005 dex higher than the metallicity of component B.
The abundance differences range from 0.03 dex for volatiles (i.e. elements such
as carbon and oxygen), and up to 0.06 dex for refractories (i.e. elements such
as iron, vanadium and magnesium). The lower abundance of heavy elements in
component B is likely due to the formation of the giant planet that is
presently in orbit around it, where the “missing” heavy elements were used to
form the giant planet.

The higher deficiency in refractories compared to volatiles
in component B means that the giant planet, 16 Cygni Bb, has a corresponding
excess of refractories. This suggests that 16 Cygni Bb formed by the core
accretion mechanism where an initial rocky core, comprised primarily of
refractories, becomes massive enough to start accreting hydrogen, helium and
other volatiles to form a giant planet. Estimates of the initial rocky core of
16 Cygni Bb places it at around 1.5 to 6 times the mass of Earth, consistent
with Jupiter’s core mass. These findings validate the core accretion model for
the formation of 16 Cygni Bb and offer yet another means to examine the
relationship between a star and its planet.